Add a debugging option to stress-test outfuncs.c and readfuncs.c.

In the normal course of operation, query trees will be serialized only if
they are stored as views or rules; and plan trees will be serialized only
if they get passed to parallel-query workers.  This leaves an awful lot of
opportunity for bugs/oversights to not get detected, as indeed we've just
been reminded of the hard way.

To improve matters, this patch adds a new compile option
WRITE_READ_PARSE_PLAN_TREES, which is modeled on the longstanding option
COPY_PARSE_PLAN_TREES; but instead of passing all parse and plan trees
through copyObject, it passes them through nodeToString + stringToNode.
Enabling this option in a buildfarm animal or two will catch problems
at least for cases that are exercised by the regression tests.

A small problem with this idea is that readfuncs.c historically has
discarded location fields, on the reasonable grounds that parse
locations in a retrieved view are not relevant to the current query.
But doing that in WRITE_READ_PARSE_PLAN_TREES breaks pg_stat_statements,
and it could cause problems for future improvements that might try to
report error locations at runtime.  To fix that, provide a variant
behavior in readfuncs.c that makes it restore location fields when
told to.

In passing, const-ify the string arguments of stringToNode and its
subsidiary functions, just because it annoyed me that they weren't
const already.

Discussion: https://postgr.es/m/17114.1537138992@sss.pgh.pa.us

src/backend/nodes/read.c

@@ -28,18 +28,30 @@
 
 
 /* Static state for pg_strtok */
-static char *pg_strtok_ptr = NULL;
+static const char *pg_strtok_ptr = NULL;
+
+/* State flag that determines how readfuncs.c should treat location fields */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+bool		restore_location_fields = false;
+#endif
 
 
 /*
  * stringToNode -
- *	  returns a Node with a given legal ASCII representation
+ *	  builds a Node tree from its string representation (assumed valid)
+ *
+ * restore_loc_fields instructs readfuncs.c whether to restore location
+ * fields rather than set them to -1.  This is currently only supported
+ * in builds with the WRITE_READ_PARSE_PLAN_TREES debugging flag set.
  */
-void *
-stringToNode(char *str)
+static void *
+stringToNodeInternal(const char *str, bool restore_loc_fields)
 {
-	char	   *save_strtok;
 	void	   *retval;
+	const char *save_strtok;
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	bool		save_restore_location_fields;
+#endif
 
 	/*
 	 * We save and restore the pre-existing state of pg_strtok. This makes the
@@ -51,13 +63,45 @@ stringToNode(char *str)
 
 	pg_strtok_ptr = str;		/* point pg_strtok at the string to read */
 
+	/*
+	 * If enabled, likewise save/restore the location field handling flag.
+	 */
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	save_restore_location_fields = restore_location_fields;
+	restore_location_fields = restore_loc_fields;
+#endif
+
 	retval = nodeRead(NULL, 0); /* do the reading */
 
 	pg_strtok_ptr = save_strtok;
 
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+	restore_location_fields = save_restore_location_fields;
+#endif
+
 	return retval;
 }
 
+/*
+ * Externally visible entry points
+ */
+void *
+stringToNode(const char *str)
+{
+	return stringToNodeInternal(str, false);
+}
+
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+
+void *
+stringToNodeWithLocations(const char *str)
+{
+	return stringToNodeInternal(str, true);
+}
+
+#endif
+
+
 /*****************************************************************************
  *
  * the lisp token parser
@@ -104,11 +148,11 @@ stringToNode(char *str)
  * code should add backslashes to a string constant to ensure it is treated
  * as a single token.
  */
-char *
+const char *
 pg_strtok(int *length)
 {
-	char	   *local_str;		/* working pointer to string */
-	char	   *ret_str;		/* start of token to return */
+	const char *local_str;		/* working pointer to string */
+	const char *ret_str;		/* start of token to return */
 
 	local_str = pg_strtok_ptr;
 
@@ -166,7 +210,7 @@ pg_strtok(int *length)
  *	  any protective backslashes in the token are removed.
  */
 char *
-debackslash(char *token, int length)
+debackslash(const char *token, int length)
 {
 	char	   *result = palloc(length + 1);
 	char	   *ptr = result;
@@ -198,10 +242,10 @@ debackslash(char *token, int length)
  *	  Assumption: the ascii representation is legal
  */
 static NodeTag
-nodeTokenType(char *token, int length)
+nodeTokenType(const char *token, int length)
 {
 	NodeTag		retval;
-	char	   *numptr;
+	const char *numptr;
 	int			numlen;
 
 	/*
@@ -269,7 +313,7 @@ nodeTokenType(char *token, int length)
  * this should only be invoked from within a stringToNode operation).
  */
 void *
-nodeRead(char *token, int tok_len)
+nodeRead(const char *token, int tok_len)
 {
 	Node	   *result;
 	NodeTag		type;